Research paperNear-infrared spectroscopy of the Chaldaea asteroid family: Possible link to the Klio family
Introduction
A primitive asteroid is one that has been minimally processed and therefore maintains a composition similar to that with which it formed; hence the study of primitive asteroids can tell us about the composition of the early Solar System. Campins et al. (2018) define primitive asteroids as those with low reflectivity (geometric albedo pV ≤ 0.15) and mostly featureless spectra. Signatures of aqueous alteration can be seen in the reflectance spectra of primitive asteroids through absorption features near 0.7 and 3 μm.
Evidence suggests that asteroids initially formed big through a fast accretion mechanism (e.g., Dermott et al., 2018; Delbo et al., 2019; Morbidelli et al., 2009). When these primordial asteroids suffered catastrophic collisions, they broke up into smaller asteroids that share similar orbital elements, called asteroid families. Depending on the composition of the parent body (i.e., differentiated or not), the resulting family members may share similar compositions or may provide us with a sample of the interior of the parent body. Over time, the orbits of family members will be altered by effects such as the Yarkovsky/YORP effects, the hypothesized giant planet migration, and subsequent, sometimes catastrophic, collisions. Evidence of further catastrophic collisions can be seen throughout the current asteroid belt. As an example, within the Themis family there is a subfamily called Beagle that is much younger than Themis (Nesvorný et al., 2008). Therefore, the current identified families may be generations removed from the original primordial planetesimals, and it is possible that multiple families could have originated in a single primordial progenitor body.
The PRIMitive Asteroid Spectroscopic Survey (PRIMASS) is a spectroscopic survey in both the visible and near infrared (NIR) that aims to characterize primitive asteroids in the main belt. By combining our spectra with geometric albedos and laboratory spectra of carbonaceous chondrites we explore the differences between and within asteroid families, test for space weathering effects through comparison of older and younger families, and obtain constraints on asteroid composition. Most of our work has focused on primitive asteroids located in the inner main belt (IMB) between 2.15 and 2.5 AU. At least eight primitive asteroid families have been identified in the IMB (Fig. 1): Polana-Eulalia (hereafter Polana), Clarissa, Sulamitis, Erigone, Klio, Chaldaea, Chimaera, and Svea (Nesvorny et al., 2015; Walsh et al., 2013). PRIMASS has observed and characterized the Polana and Klio families in the visible and NIR (Arredondo et al., 2020; de León et al., 2016; Morate et al., 2019; Pinilla-Alonso et al., 2016) and the other six families in the visible only (Morate et al., 2016, Morate et al., 2018, Morate et al., 2019).
For primitive asteroids, it was expected that family members should look spectrally similar; however, we have found that this is not always the case. A summary of previous PRIMASS results is given in Table 1. We find that primitive IMB asteroid families differ based on the dispersion of spectral slopes (homogeneous vs diverse) and the percentage of objects that show the 0.7 μm absorption feature associated with hydration. This work focuses on the Chaldaea family, which in the visible has homogeneous spectral slopes and 79% of the sample from Morate et al. (2019) showed evidence of hydration at 0.7 μm. The Chaldaea family has 132 identified members, 28 of which have SMASS taxonomies (96% C-type and 4% X-type). The mean albedo for the entire family is 0.07 ± 0.03 and 85% of family members have pV < 0.1 (Mainzer et al., 2019).
Morate et al. (2019) present an interesting hypothesis about the relationship between the Chaldaea family and the nearby Klio family. They posit that because of their similar location and complementary compositions (Chaldaea is hydrated but homogeneous, Klio is diverse but significantly less hydrated), Chaldaea and Klio may have formed from one large primordial object, with the more hydrated Chaldaea objects near to the surface and the less hydrated Klio objects in the inner layers. It is also possible that the contrary is true, that Klio are the objects from the outside layers and the more hydrated Chaldaea objects are the inside layers. Young et al. (1999) use oxygen-isotope compositional data to infer that CI, CM, and CV chondrites could have been derived from different zones in a single body, with the more altered CI objects nearer to the center. In this case, the Chaldaea objects would be the CI analog that would have made up the inner layers of the body, while the less hydrated Klio objects would be located closer to the surface.
Morate et al. (2019) present a Yarkovsky cone for the combined Chaldaea-Klio family, which shows the maximum distance that family members will drift over the age of the family due to the Yarkovsky effect (Fig. 2). This cone enveloped most objects from both families including the two parent bodies of each family. We note that there is not a clear explanation for why the Chaldaea objects seem to cluster so tightly to one side of the parent body while the Klio objects are spread evenly on both sides. It could be that they are majority prograde rotators and therefore only drift outward from the Sun or that the higher inclination Chaldaea objects are closer to the ν6 resonance and therefore are depleted at lower semimajor axes. The common origin hypothesis of Morate et al. (2019) is supported by the small difference in mean visual slope of the two families (Chaldaea mean: 0.88 ± 1.25%/1000 Å, Klio mean: 1.16 ± 2.04%/1000 Å). Because the outer layer objects would be more exposed to space weathering, they may be more altered in slope.
The goal of this paper is to use our NIR spectroscopy to characterize the Chaldaea family and to test the link to the Klio family proposed by Morate et al. (2019). Details of the observations and the data reduction process are given in Section 2. The analysis of the computed reflectance spectra is shown in Section 3. We compare our spectra with the Klio family and discuss implications in Section 4. Conclusions and future work are given in Section 5.
Section snippets
Observations and data reduction
The Chaldaea family has 132 objects identified by Nesvorny et al. (2015). We did not impose any observing constraints (i.e., low magnitude or low albedo) on our targets because this is a small and faint family. We did, however, prioritize observing objects with visible spectra published in Morate et al. (2019). Our sample includes 15 Chaldaea objects (11% of the total members) including asteroid (313) Chaldaea, the likely parent body of the family. Two of the objects were observed twice and one
Characteristics of the sample
The mean albedo of our sample is 0.08 ± 0.04 which is slightly higher than the average for the whole family and might be due to our observing bias for brighter objects. The diameters of objects in our sample range from 4 to 96 km with a median diameter of 7 km. We identified one interloper object based on the shape of its spectrum and its taxonomic type, (7030) Colombini, which we discuss more in Section 4.3. We exclude this non-primitive asteroid from the rest of our analysis.
The probable
Spectral similarities with Klio – one common parent body?
The results of PRIMASS studies find a clear trend with composition and inclination in the lower-inclination families. The Polana and Clarissa families are homogeneous, anhydrous and cluster tightly around 3°, and the Erigone and Sulamitis families are heterogeneous, hydrated and cluster around 5° (Table 1). This led us to consider that the families at similar inclinations could be linked. Our working hypothesis (Lowry, 2018) is that the parent bodies of families with similar spectra and
Summary and conclusion
In the NIR, the Chaldaea family is spectrally homogeneous, red sloped, and concave shaped. Our taxonomy determination using NIR and VNIR spectra shows that the Chaldaea family is dominated by C-type asteroids and that B-type asteroids are not common. In comparison with the similar inclination Klio family, the Chaldaea family has essentially the same average slope, albedo, and slope dispersion. The major difference between the Klio and Chaldaea families is that the Chaldaea family presents very
Declaration of Competing Interest
None.
Acknowledgements
Support for this work was provided by NASA grant NNX17AG92G. Parts of these data were obtained as visiting astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration. This work is based in part on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma, Spain by the Fundación Galileo Galilei of the INAF (Instituto Nazionale di
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